Microscope transmitted-illumination apparatus

ABSTRACT

In the present invention, a transmission-illumination apparatus for use in a microscope has a transmission-illumination optical system comprising a condenser lens for radiating light emitted from a light source onto a sample, and an observation optical system including an objective lens for observing the sample. At least two shield members for controlling a shape of an aperture created in a pupil of the objective lens are disposed at or near positions conjugate with a pupil position of the objective lens. Thereby, a contrast can be successively varied without disposing a purpose-specific optical element, etc. in the observation optical system, and illumination with an optimal contrast can be provided for various samples.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 09/514,863 filed on Feb.28, 2000 now U.S. Pat. No. 6,396,628, which is a continuation ofInternational Application No. PCT/JP98/03853, filed Aug. 28, 1998, theentire disclosures of which are incorporated hereby reference.

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 09-234070, filed Aug. 29,1997; No. 09-234784, filed Aug. 29, 1997; and No. 10-240403, filed Aug.26, 1998, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates to a transmitted-illumination apparatusapplicable to various types of microscopes.

There are conventional methods, such as a phase-contrast observationmethod, a differential-interference observation method, amodulation-contrast method and an oblique illumination method, forvisualizing various colorless transparent phase-samples and observingthem.

In the phase-contract observation method, a ring slit is provided at aposition of a pupil of an illumination optical system of a microscope. Aphase film having a conjugate shape with the ring slit is disposed at apupil of a focusing optical system provided at a position conjugate withthe ring slit.

An advantage of this observation method resides in that observationimages with clear contrast can be obtained with high detectionsensitivity, even for samples with a small difference in refractiveindex between structures, or minute granular structures of cells. On theother hand, a disadvantage of this observation method resides in that aphenomenon called “halo”, in which an end portion of a structure of asample looks shining in white, occurs and this makes it difficult todetermine the contour of a structure. In addition, it is necessary thatthe ring slit provided in the illumination optical system and the phasefilm disposed at the pupil plane of the observation optical system bemade to coincide by projection, thereby improving the aberrationperformance of the pupil from the ring slit to the phase film plane. Inthe phase-contrast observation method, there arises no problem with theobservation at a high magnification, but the aberration performance ofthe pupil for the observation at a low magnification or a very lowmagnification cannot satisfactorily be corrected. In fact, thephase-contrast observation method is applicable to objective lenses witha magnification of ×4 at most.

In the differential-interference observation method, two polarized lightcomponents crossing at right angles, which are produced by abirefringent crystal, are radiated on a sample plane with a slightdisplacement, and these light components are made to interfere with eachother, thereby observing a minute structure of the sample. An advantageof this observation method resides in that stereoscopic observation withvery high contrast can be performed. On the other hand, a disadvantageof this observation method is that the use of the birefringent crystalincreases costs and because of use of polarized light, no exactobservation image can be obtained in a case of a material which affectsthe polarized state. For example, a plastic Petri dish is unsuitable forthe differential-interference observation. The reason is that polarizedlight is disturbed by birefringence of plastic material. In addition,the polarization state is disturbed by a distortion of a lens or anobjective lens in the illumination optical system, a purpose-specificobjective lens, etc. is needed. Moreover, since two light beams aresubjected to interference, a lens capable of actual observation needs tohave a magnification of ×4 or more, and this is not suitable forobservation with a low magnification or a vary low magnification.

In the modulation-contrast observation method, as disclosed in Jpn. Pat.Appln. KOKAI Publication No. 51-128548, a slit is provided at a positionof a pupil of an illumination optical system of a microscope, and aplurality of regions with different transmittances are provided at aposition of a pupil of a focusing optical system. Normally, anabsorption film having a proper transmittance is disposed at a regionconjugate with the slit. A transmission region is provided on one sideadjacent to the absorption film, and a light-shield region is providedon the other side. On a pupil plane, light transmission regions varydepending on a magnitude of refraction due to a structure in a sample,and a transmittance varies accordingly. Thus, a stereoscopic image withwhite/black shading can be obtained. An advantage of this observationmethod resides in that a stereoscopic image with shading on a phaseobject can be obtained with a relatively inexpensive structure. Sincethis method is free from halo, which occurs in the above-mentionedphase-contrast observation method, the contour of a structure can beeasily observed and this method is suitable for manipulation of a cell,etc. On the other hand, a disadvantage of this observation methodresides in that the detection sensitivity is lower than in thephase-contrast observation method and it is difficult to determine aminute structure. Moreover, a difficult operation for regulating thedirections of the slit and absorption film needs to be performed eachtime the objective lens is exchanged. Besides, in order to project theslit onto the absorption film of the observation optical system, it isnecessary to improve the aberration of the optical system for projectingthe pupil, like the phase-contrast observation method. Because of this,with the objective lens of a low magnification or a very lowmagnification, the pupil aberration cannot satisfactorily be correctedand proper observation cannot be performed.

There are an oblique illumination method and a dark-field illuminationmethod as illumination methods for visualizing phase-samples.

FIGS. 1A to 1D are schematic views of condenser lenses in generaloblique illumination methods. In these figures, numeral 1 denotes anaperture stop; 2 a, 2 b lens groups; and 3 a sample. The aperture stop 1limits the aperture for illumination and has a variable circularaperture. The aperture stop 1 moves in a plane perpendicular to anillumination optical axis O, thereby controlling the angle ofillumination onto the sample 3. Specifically, FIG. 1B shows the state ofthe pupil in a case where the aperture stop 1 in the state shown in FIG.1A has been moved and reduced. FIG. 1C shows the state of the pupil in acase where the aperture stop 1 has been further reduced. FIG. 1D showsthe state of the pupil in a case where the aperture stop 1 has beenshifted while being opened.

FIG. 2A is a schematic view of a condenser lens in a general dark-fieldillumination method. In the conventional dark-field illumination method,as shown in the figure, a stop 1 a, which has an inside portion shut offand has an outside annular portion provided with a slit, is disposednear a location where an aperture stop is disposed. As is shown in FIG.2B, the stop 1 a has a central light-shield region 1 b. The region 1 bprevents illumination light from directly entering an objective lens. Inaddition, scattered light from the sample 3 is observed to realizedark-field observation. In this case, the shape of the stop 1 a isselected in accordance with the numerical aperture of the objectivelens, whereby dark-field observation can be made using various objectivelenses.

As regards observation using microscopes, not only micro-regions butalso macro-regions need to be observed. There are cases where the use ofan objective lens with a magnification of ×1, an objective lens with avery low magnification of ×0.5, etc. is desired. In general, astereomicroscope is used for observing such macro-regions. Thestereomicroscope is advantageous in that the cost is low, theoperability is high and stereoscopic observation can be performed. Inaddition, as regards illumination methods, there are means, such asdark-field illumination, bright-field illumination and obliqueillumination, for visualizing transparent samples such as phase samples.

Jpn. Pat. Appln. KOKAI Publication No. 4-318804 discloses atransmission-illumination apparatus for a stereomicroscope, whichpermits oblique illumination. FIG. 3A shows thetransmission-illumination apparatus disclosed in this publication. As isshown in FIG. 3A, this apparatus is constructed such that light from alight source 5 is guided to a mirror 8 via a collector lens 6 and afrosted glass 7, and a light beam reflected by the mirror 8 is radiatedvia a condenser lens 9 onto a sample 10 a placed on a sample-mountingtransparent member 10 and then guided to an objective lens 12. Byrotating the mirror 8 and changing the angle thereof, the ratio betweena dark portion 13 a and a bright portion 13 b of a pupil 13 of each ofright and left objective lenses, as shown in FIG. 3B, can be controlled.

Jpn. U.M. Appln. KOKOKU Publication No. 41-5808 discloses atransmission-illumination apparatus for a stereomicroscope capable ofselectively effecting oblique illumination and dark-field illumination.FIGS. 4A and 4B are views for describing this apparatus. As is shown inFIG. 4A, this apparatus is constructed such that light from a lightsource 5 is guided to a mirror 8 via a collector lens 6 and a frostedglass 7, and light reflected by a mirror 8 is radiated via a condenserlens 9 onto a sample 10 a and then guided to an objective lens 12. Aknife edge 15 for cutting a light beam is provided near the frostedglass 7 disposed at a position conjugate with the pupil of the objectivelens 12.

As is shown in FIG. 4B, the knife edge 15 is vertically moved relativeto a conjugate image 17 of the pupils of the two juxtaposed objectivelenses, whereby oblique illumination and dark-field illumination isselectively effected. The aforementioned Jpn. Pat. Appln. KOKAIPublication No. 4-318804 proposes that a stop be substituted for theknife edge 15 shown in FIG. 4A.

A purpose-specific observation optical system is required for theabove-described phase-contrast observation method,differential-interference observation method and modulation-contrastobservation method which can perform observation of a transparent objectsuch as a phase sample. It is also necessary, for example, to correctthe optical performances of the illumination optical system and thepupil projection optical system of the observation optical system. Thus,these methods are not suitable for observation with a low magnificationor a very low magnification.

In the oblique-illumination method shown in FIG. 1A, if the aperturestop 1 is shifted and reduced, as shown in FIG. 1C, the resolution andthe luminance of illumination light become deficient. If the aperturestop 1 is shifted, as shown in FIG. 1D, it becomes difficult to controlthe degree of freedom of oblique illumination, i.e. the ratio betweenillumination light directly incident on the objective lens andnon-incident illumination light. The reason is that the aperture stop isconstructed to form a circular opening.

In addition, in the dark-field illumination illustrated in FIG. 2, theangle of dark-field illumination light varies depending on the width ofthe annular slit or the position of the aperture. Consequently, if thethickness, etc. of the sample varies, the sample cannot be made visiblewith good contrast. Specifically, in order to freely control the angleof illumination light, it is necessary to prepare many annular slitswith different structures, and this is not practical.

Moreover, as regards the oblique illumination method proposed in theabove-described stereomicroscope, only one of the pupils of the rightand left objective lenses is illuminated. Thus, only one kind ofcontrast is obtained. Although the effect of oblique illumination can beobtained by disposing the slit at the pupil of the illumination opticalsystem and thereby restricting the aperture of the pupil of theobjective lens, the shape of the slit or the position of the slit isfixed in the prior art. It is thus not possible to freely and finelycontrol the intensity of illumination light or the angle ofillumination, depending on the thickness and refractive index of varioussamples.

As has been described above, with the conventional illuminationapparatus for microscopes, phase-samples cannot satisfactorily be madevisible with high contrast in observation with a low or very lowmagnification.

Recently, stereomicroscopes have been constructed as systems, and a widerange of magnification is required. In addition, high operability isrequired. In order to meet a demand for use with a wide range ofmagnification, it is necessary to achieve uniform illumination over awide visual field. In view of easier use, a sample plane needs to besituated at a level as low as possible.

In the above-described prior art, the frosted glass (diffusion plate)needs to be enlarged in order to increase the visual field, and thedeflecting mirror, too, needs to be enlarged. Because of this, thethickness of the illumination optical system increases, and both ademand for a wider visual field and a demand for a low-level sampleplane cannot be satisfied.

Jpn. U.M. Appln. KOKOKU Publication No. 45-1105 discloses anillumination apparatus capable of performing bright-field illuminationand dark-field illumination, as shown in FIG. 5. In this illuminationapparatus, a light source 100 is disposed under an objective lens 101and a sample 102. In the dark-field illumination mode, a shutter 103 isclosed to shut off direct light traveling to the sample 102. Inaddition, light from the light source 100 is reflected by a cylindricalmirror 105 and made obliquely incident on the sample 102. In thebright-field illumination mode, the shutter 103 is opened and light fromthe light source 100 is made directly incident on the sample.

In this illumination apparatus, however, the light source is disposedvertical to the sample. Consequently the optical path is short and, nospace is left for mounting optical members such as a filter. If anoptical member is to be disposed on the optical path, the thickness ofthe apparatus with this structure is increased. Furthermore, since theoptical path is short, the wide visual field cannot uniformly beilluminated.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an illuminationapparatus for a microscope, wherein a phase-sample is visualized with agood contrast without disposing a purpose-specific optical element, etc.in an observation optical system, in particular, in a region of a lowmagnification to a very low magnification, and a structure and adistribution thereof can be specified. Specifically, atransmission-illumination apparatus is provided wherein a contrast issuccessively varied for various samples with different thicknesses andrefractive indices and optimal illumination is performed for thesamples.

Another object of the invention is to provide atransmission-illumination apparatus wherein a sample-mounting surfacecan be set at a low level, that is, a height between a bottom surface ofa microscope body and the sample-mounting surface can be reduced.

Still another object of the invention is to provide atransmission-illumination apparatus wherein a bright-field opticalsystem and a dark-field optical system can be switched to observe asample and a height between a bottom surface of a microscope body and asample-mounting surface can be reduced.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1A to FIG. 1D schematically show condenser lenses in generaloblique-illumination methods and aperture shapes of pupils created byaperture stops thereof;

FIG. 2A schematically shows a condenser lens in a general dark-fieldillumination method, and FIG. 2B shows a shape of a stop;

FIG. 3A shows a schematic structure of a conventionaltransmission-illumination apparatus, and

FIG. 3B shows aperture shapes of pupils of right and left objectivelenses;

FIG. 4A shows a schematic structure of another conventionaltransmission-illumination apparatus, and

FIG. 4B shows a relationship between a knife edge and pupils of rightand left objective lenses;

FIG. 5 shows a conventional microscope wherein a bright-fieldillumination apparatus and a dark-field illumination apparatus can beswitched;

FIG. 6 shows an external appearance of a stereomicroscope to which atransmission-illumination apparatus according to the present inventionis applicable;

FIG. 7 shows a schematic structure of an optical system of thestereomicroscope shown in FIG. 6;

FIG. 8A schematically shows portions of a condenser lens and anobjective lens in a bright-field illumination state, and

FIG. 8B shows a state of a pupil at that time;

FIG. 9A schematically shows portions of a shield member, a condenserlens and an objective lens in the optical system shown in FIG. 7, and

FIG. 9B shows a state of a pupil at that time;

FIG. 10A shows a state in which the shield member has been shifted inthe structure shown in FIG. 9A, and FIG. 10B shows a state of a pupil atthat time;

FIG. 11A shows a state in which the shield member has been shifted inthe structure shown in FIG. 9A, and FIG. 11B shows a state of a pupil atthat time;

FIG. 12A to FIG. 12D correspond to FIG. 8B to FIG. 11B and showpositional relationships between the pupil and shield member in the casewhere the optical system shown in FIG. 7 is applied to thestereomicroscope;

FIG. 13 shows an example of a shield member driving mechanism providedwithin the microscope body;

FIG. 14 shows a second example of the structure of the shield memberdriving mechanism;

FIGS. 15A and 15B show a third example of the structure of the shieldmember driving mechanism, FIG. 15A being a plan view, FIG. 15B being aside view;

FIGS. 16A and 16B show a forth example of the structure of the shieldmember driving mechanism, FIG. 16A being a plan view, FIG. 16B being across-sectional view taken along line XVIB—XVIB in FIG. 16A;

FIG. 17A and FIG. 17B show a structure for partly controlling lightintensity for an aperture created at the pupil of the objective lens,FIG. 17A schematically showing an optical system, FIG. 17B showing arelationship between the shield member and the pupil, and FIG. 17C showsanother example of the structure of the shield member portion;

FIG. 18A and FIG. 18B show another example of the structure for partlycontrolling light intensity for an aperture created at the pupil of theobjective lens;

FIG. 19A and FIG. 19B show another example of the structure forcontrolling an aperture shape created at the pupil of the objectivelens, FIG. 19A schematically showing an optical system, FIG. 19B showinga relationship between the shield member and the pupil;

FIG. 20A and FIG. 20B show another example of the structure forcontrolling an aperture shape created at the pupil of the objectivelens, FIG. 20A schematically showing an optical system, FIG. 20B showinga relationship between the shield member and the pupil;

FIG. 21 shows a structure for partly controlling light intensity for anaperture created at the pupil of the objective lens in thestereomicroscope, as well as showing a relationship between the shieldmember and the pupil;

FIG. 22 shows another example of the structure of the shield memberportion in FIG. 21;

FIG. 23 shows another example of the structure for partly controllinglight intensity for an aperture created at the pupil of the objectivelens;

FIG. 24 shows another example of the structure for partly controllinglight intensity for an aperture created at the pupil of the objectivelens;

FIG. 25A and FIG. 25B show another example of the structure forcontrolling an aperture shape created at the pupil of the objective lensin the stereomicroscope, FIG. 25A schematically showing an opticalsystem, FIG. 25B showing a relationship between the shield member andthe pupil;

FIG. 26A and FIG. 26B show another example of the structure forcontrolling an aperture shape created at the pupil of the objective lensin the stereomicroscope, FIG. 26A schematically showing an opticalsystem, FIG. 26B showing a relationship between the shield member andthe pupil;

FIG. 27A to FIG. 27E show another example of the structure of the shieldmember, each showing an example of positional relationship in a casewhere two shield members have been shifted;

FIG. 28A and FIG. 28B show another example of the structure of theshield member, each showing an example of positional relationship in acase where two shield members have been shifted;

FIG. 29A to FIG. 29C show another example of the structure of the shieldmember, FIG. 29A showing a structure with one shield member, FIG. 29Band FIG. 29C showing examples of positional relationship in a case wheretwo shield members have been shifted;

FIG. 30 shows another example of the structure of thetransmission-illumination optical system;

FIG. 31 shows the structure of a high-magnification condenser lens foruse in the transmission-illumination optical system according to thepresent invention;

FIG. 32 shows a second structure of the high-magnification condenserlens;

FIG. 33 shows the structure of a low-magnification condenser lens foruse in the transmission-illumination optical system according to thepresent invention;

FIG. 34A and FIG. 34B show structures of switchable condenser lenses foruse in a microscope transmission-illumination apparatus, FIG. 34Ashowing a structure of a high-magnification condenser lens, FIG. 34Bshowing a structure of a low-magnification condenser lens;

FIG. 35A and FIG. 35B show second structures of switchable condenserlenses for use in the microscope transmission-illumination apparatus,FIG. 35A showing a structure of a high-magnification condenser lens,FIG. 35B showing a structure of a low-magnification condenser lens;

FIG. 36 shows a second embodiment of the present invention;

FIG. 37 shows a modification of the embodiment shown in FIG. 36;

FIG. 38A shows a third embodiment of the microscopetransmission-illumination apparatus according to the present inventionand, in particular, a bright-field optical system, and FIG. 38B shows ashape of a convex lens;

FIG. 39A is a cross-sectional view showing, in particular, a dark-fieldoptical system in the structure shown in FIG. 38A, and FIG. 39B shows ashape of a convex lens;

FIG. 40 shows an optical system switching mechanism;

FIG. 41 shows a filter insert/remove mechanism;

FIGS. 42A and 42B are views for describing the operational advantage ofthe third embodiment;

FIGS. 43A and 43B show a modification of the filter insert/removemechanism;

FIG. 44A shows a first modification of the third embodiment and, inparticular, a bright-field optical system, and

FIG. 44B shows a shape of a convex lens;

FIG. 45 shows an optical system switching mechanism in the firstmodification; and

FIG. 46 shows a filter switching mechanism in the first modification.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described by referringto ordinary microscopes as examples.

FIG. 7 schematically shows a structure of an optical system of amicroscope. The optical system comprises a transmission-illuminationoptical system for illuminating a sample, and an observation opticalsystem for observing the sample.

The transmission-illumination optical system comprises a light source 20such as a halogen lamp; a collector lens 21 for converting light fromthe light source 20 into a substantially parallel light beam; adiffusion plate (frosted glass) 22 for diffusing light from thecollector lens; a field stop 23 for limiting the light beam emanatingfrom the diffusion plate; a deflection mirror 24 for deflecting upwardthe light beam which has passed through the field stop; a projectionlens 25 for projecting a light-source image from the deflection mirror;and a condenser lens 26 for converting the light from the projectionlens into a substantially parallel beam and illuminating a sample 30. Inthis case, the light-source image from the projection lens 25 isprojected onto a pupil position P1 of the transmission-illuminationoptical system, which is located at a front-side focal position of thecondenser lens 26, thereby illuminating the sample 30.

The observation optical system comprises an objective lens 31, afocusing lens 32 and an eyepiece lens 33. A focal position P2 of theobjective lens 31, which is a pupil position of the observation opticalsystem, is conjugate with the pupil position P1 which is the focalposition of the condenser lens. The light which has passed through thesample 30 is observed by means of the eyepiece lens 33 via the objectivelens 31 and focusing lens 32.

In the transmission-illumination optical system with the abovestructure, at least two shield members 40 a and 40 b are disposed to beindependently movable at a position (i.e. pupil position P1) conjugatewith the pupil position P2 of the objective lens 31 or near theconjugate position. By shifting the shield members 40 a, 40 b, the shapeof the aperture created within the pupil of the objective lens 31 iscontrolled, as will be described later in detail. Specifically, theangle of illumination light for illuminating the sample 30 can be variedto control the ratio between the illumination light directly enteringthe objective lens 31 and the diffraction light emanating from thesample 30.

This will be described specifically with reference to FIGS. 8A to 11B.In these figures, FIGS. 8A, 9A, 10A and 11A schematically show opticalsystems, and FIGS. 8B, 9B, 10B and 11B show positional relationshipsbetween pupils and shield members in microscopes. A circle indicated bysymbol A represents a pupil with a maximum numerical aperture at whichthe condenser lens 26 can perform illumination. A circle indicated bysymbol C represents a pupil corresponding to a numerical aperture of theobjective lens 31 of the microscope at the pupil position P1 of thecondenser lens 26. A circle indicated by symbol B represents a lightincidence state (aperture shape; a black portion being shielded byshield member 40 a, 40 b) at the pupil position of the objective lens31.

FIGS. 8A and 8B indicate states in which no shield member is present,that is, so-called bright-field illumination states. In this structure,at least two shield members 40 a and 40 b, as shown in FIG. 7, aredisposed to be independently movable at a pupil position P1 of thecondenser lens 26, or near this position (± several mm from pupilposition P1 in the optical axis direction). In this case, aperture stop41 may be disposed adjacent to the shield members.

There are at least two shield members. The shape of each shield member,the number of shield members, and the method of movement thereof can bevariously determined. For example, as shown in FIG. 8A, each shieldmember may have a rectangular shape, and the shield members may beshifted independently, as indicated by arrows, to shut off the pupil C.Specifically, if the shield members 40 a and 40 b are positioned, asshown in FIG. 9A and FIG. 9B, the aperture shape (illumination state)represented by pupil B is obtained. In the figures, a region (hatched)shut off by the shield member 40 a corresponds to a left-side blackregion in the pupil B, and a region shut off by the shield member 40 bcorresponds to a right-side black region in the pupil B.

FIG. 10B shows a state in which the shield member 40 b has been furthershifted toward the shield member 40 a from the state shown in FIG. 9Band the distance therebetween narrowed. As is indicated by the apertureshape in the pupil B in this state, the illumination light directlyentering the sample 30 is only that in the narrow region (the narrowupper region of the filed of view when observation is performed using amicroscope shown in FIG. 6) in the left side of the pupil B in thefigure. The effect of oblique illumination is obtained by independentlyshifting the shield members 40 a and 40 b, and the effect of theluminance stop is obtained by varying the distance between the shieldmembers 40 a and 40 b. The shape of the aperture created in the pupil ofthe objective lens can be controlled by freely shifting the shieldmembers 40 a and 40 b. Specifically, since the angle and amount ofillumination light for illuminating the sample 30 are successivelyvaried, the ratio in intensity between the illumination light directlyentering the objective lens and the diffraction light emanating from thesample can be successively controlled. Optimal observation can beperformed according to the sample.

If the shield members 40 a and 40 b are shifted to shut off the pupil C,as shown in FIG. 11B, direct light incident on the objective lens can becut to achieve dark-field illumination for observing scattered lightfrom the sample. In this case, the amount and angle of dark-fieldillumination light can be controlled by varying the distance between theshield members and the position of each shield member in the state inwhich the pupil C is shut off.

The above optical system is applicable to a stereomicroscope. FIGS. 12Ato 12D correspond to FIGS. 8B, 9B, 10B and 11B and show positionalrelationships between the pupils and shield members in the case wherethe above-described optical system is applied to a stereomicroscope. InFIGS. 12A to 12D, circles denoted by symbols C1 and C2 represent pupilscorresponding to the apertures of the right and left objective lenses ofthe stereomicroscope. Circles denoted by symbols B1 and B2 representlight incidence states at the pupil positions of the right and leftobjective lenses.

A description will now be given of an example of the structure whereinthe transmission-illumination apparatus according to the presentinvention has been applied to the stereomicroscope. FIG. 6 is a sideview of the entire structure of the stereomicroscope. Thestereomicroscope 200 comprises a transmission-illumination frame 205having a lever 210 a (210 b) (to be described later) for switchingshield members, filter levers 202, a volume dial 203 and amirror-inclination adjustment lever 204 which is provided wherenecessary; a lamp house LH; a focusing section F; a focusing handle FH;a lens-barrel K; a lens body KB; an objective lens receiver T; andeyepiece lenses EO. A sample S is placed on the surface of thetransmission-illumination frame and it is observed by means of two rightand left eyepiece lenses EO.

A driving mechanism for the shield members 40 a and 40 b will now bedescribed with reference to FIGS. 13 to 16.

As is shown in FIG. 13, shield members 40 a and 40 b are attached toinside portions of distal end portions of two slidable levers 210 a and210 b, which are individually penetrated through a wall of a frame body205 a of the transmission-illumination frame 205 shown in FIG. 6. Thelevers 210 a and 210 b are independently operable in directions ofarrows. Thereby, as shown in FIGS. 9 to 11, the shield members 40 a and40 b can be moved in a direction (a front-and-rear direction in respectof the observer) perpendicular to a direction parallel to a planeincluding optical axes of the right and left observation opticalsystems. A translation mechanism (not shown) may be provided at rear endportions of the levers 210 a and 210 b such that both shield members areinterlocked.

FIG. 14 shows a second example of the structure of the shield membersand the driving mechanism therefor.

Shield members 40 a and 40 b are attached to inside portions of distalend portions of two levers 211 a and 211 b, which are independentlyslidable in the directions of arrows and are individually penetratedobliquely through a side wall of the frame body 205 a. Like thisexample, the operation lever for moving each shield member can beattached at a given position of the side wall of the frame body 205 a,and the shape of each shield member can be freely changed in accordancewith the position of attachment (a pentagonal shape in the figure). Inthis example of the structure, too, a translation mechanism may beprovided at rear end portions of the levers 211 a and 211 b such thatboth shield members are interlocked.

FIGS. 15A and 15B show a third example of the structure of the shieldmembers and the driving mechanism therefor. FIG. 15A is a plan view, andFIG. 15B is a side view.

As is shown in the figures, shield plates 40 a and 40 b are providedrespectively with slots 40 a 1, 40 a 2 and 40 b 1 and 40 b 2 extendingobliquely (directions Y1 and Y2) as well as slots 40 a 3 and 40 b 3extending in the right-and-left direction.

Two slidable levers 212 a and 212 b are individually penetrated througha side wall of the frame body 205 a. Pins 213 a and 213 b engaging theslots 40 a 3, 40 b 3 formed in the shield members 40 a, 40 b are formedat distal end portions of the levers. Links 215 a, 215 b, 215 c and 215d engaging the slots 40 a 1, 40 a 2 and 40 b 1, 40 b 2 formed in theshield members 40 a, 40 b are provided on a bottom surface of the framebody 205 a.

Thus, if the levers 212 a and 212 b are inserted and withdrawn in thedirections X1 and X2, the shield members 40 a and 40 b are moved in thedirections Y1 and Y2 along the slots and also moved in theright-and-left direction relative to each other.

FIGS. 16A and 16B show a forth example of the structure of the shieldmembers and the driving mechanism therefor. FIG. 16A is a plan view, andFIG. 16B is a side view. This driving mechanism includes a cam mechanismin addition to the above-described link mechanism.

An axially movable and rotatable cam shaft 220 is penetrated through aside wall of the frame body 205 a. Cam followers 221 a and 221 b towhich shield members 40 a and 40 b are attached are provided on the camshaft 220. Rotation hold shafts 222 a and 222 b are provided on the camfollowers 221 a and 221 b. One end portion of each of the rotation holdshafts 222 a and 222 b is engaged with a slot 225 a formed in a holdportion 225 of the frame body 205 a, thereby checking rotation of thecam follower 221 a, 221 b. The other end portion of each of the rotationhold shaft 222 a, 222 b is engaged with a helical groove 220 a, 220 bformed in the cam shaft 220. When the cam shaft 220 is rotated by a knob220 c, the cam followers 221 a and 221 b are axially moved.

Thus, if the cam shaft 220 is moved axially (in the direction of arrowX), the shield members 40 a and 40 b can be axially moved as one body.If the cam shaft 220 is rotated by the knob 220 c, the shield members 40a and 40 b can be moved toward, and away from, each other andaccordingly the distance therebetween can be varied.

According to the above-described shield members and driving mechanism,the pupils of the right and left objective lenses of thestereomicroscope can be uniformly reduced in the front-and-reardirection. Since the right and left pupils are uniformly reduced, theright and left images are viewed in the same manner. A stereoscopiceffect is created by a right-and-left parallax characterizing thestereomicroscope. In this case, by moving the shield members 40 a and 40b, the ratio between direct light incident on the sample and diffractionlight can be freely adjusted and the observation can be performed whilesuccessively varying contrast. Moreover, direct light can be cut off byapproaching the shield members 40 a and 40 b to each other, anddark-field observation can be realized. As described above, since theamount of light incident on the pupil of each objective lens, thecontrast can be freely controlled. In addition, since oblique light canbe added, the contrast can be further enhanced and observation forvarious samples realized. The above-described driving mechanism isapplicable to ordinary microscopes.

A description will now be given of another structure for controlling theaperture shape created within the pupil of the objective lens, byreferring to a case where the invention is applied to an ordinarymicroscope.

FIG. 17A to FIG. 17C show a structure for partly controlling lightintensity for an aperture created at the pupil of the objective lens. Asis shown in FIG. 17A, an optical member for controlling light intensity,for example, an ND (Neutral density) filter 45, is movably disposed nearone of the shield members, 40 a. The filter 45 can move in the directionof the arrow, as shown in FIG. 17B, independently from the movement ofthe shield member 40 a.

In FIG. 17B, regions where the pupil C is shielded by the shield members40 a and 40 b are indicated by hatching, and a region where light passesthrough the filter 45 is indicated by crossing lines. With thisstructure, as represented by the pupil B, regions with different amountsof light can be created in the pupil of the objective lens (in the pupilB, a region indicated by symbol D is an opening region, and a regionindicated by symbol E and crossing lines is a region at which the lightamount is reduced by the filter 45 in the opening region). As a result,the amount of illumination light directly entering the sample can bereduced, and a fine area of the sample can easily be observed with highcontrast. In addition, if the shield members 40 a, 40 b and filter 45are moved, as desired, the angle of illumination light for illuminatingthe sample is varied and the ratio between the illumination lightdirectly entering the objective lens and the diffraction light emanatingfrom the sample can be adjusted more finely.

Although the filter 45 is so disposed as to overlap the shield member 40a, it is possible to provide another filter 45 at the region of theshield member 40 b. This increases the degree of freedom ofillumination.

FIG. 18 shows another example of the structure for partly controllingthe light intensity for the aperture created at the pupil of theobjective lens. In a structure shown in FIG. 18A, two ND filters 45 aand 45 b with different light attenuation ratios are disposed in astacked manner near one shield member 40 a. The two ND filters aremovable independently from each other as well as from the shield members40 a and 40 b.

With this structure, the light intensity for the opening with the samesize can be controlled and the degree of freedom of illumination isincreased. A phase-sample, etc. can be made visible, and the contrastcan be controlled more finely. Needless to say, in this structure, too,filters 45 a and 45 b of the same structure may be disposed on the sideof the shield member 40 b.

In the above-described structure, the ND filter is used as a member forcontrolling the light intensity. However, even if a polarizing elementis used, the intensity at the aperture can be controlled as desired. Forexample, as shown in FIG. 18B, a rotatable polarizing plate 46 acovering the entire area of the pupil A of the condenser may be disposedadjacent to the shield members 40 a and 40 b. In addition, a polarizingplate 46 b is disposed near the shield member 40 a so as to be movablein the direction of the arrow. Thus, if the polarizing plate 46 b isoverlapped over the polarizing plate 46 a and the polarizing plate 46 ais rotated, the light intensity at the overlapping region can besuccessively controlled, and the contrast of the phase sample can besuccessively varied.

Even if a liquid crystal panel is substituted for the above-described NDfilter or polarizing plates, the same advantages can be obtained.Specifically, by controlling the voltage applied to the liquid crystalpanel, the shape of the aperture created in the pupil of the objectivelens can be varied and a region with different brightness can be freelycreated in the aperture. Besides, the ND filter, polarizing element andliquid crystal element may be freely combined.

In the above structure, the condenser lens includes the aperture stop41, as shown in FIG. 7. How to control the aperture created in the pupilof the objective lens in a case where the condenser lens has theaperture stop 41 will now be described with reference to FIGS. 19A and19B.

The aperture stop 41 is so constructed as to reduce the pupil A with themaximum numerical aperture at which the condenser lens 26 can performillumination. By controlling the aperture stop 41 along with the shieldmembers 40 a and 40 b, the light at regions G and H can be cut off, asshown in the figure. Specifically, the aperture region of the pupil B ofthe objective lens can be limited in the longitudinal direction bynarrowing the aperture stop 41. Moreover, dark-field illumination lightwhich does not directly enter the objective lens can be adjusted. Theprovision of the aperture stop 41 is applicable to all of theabove-described examples of the structure.

The direction of movement of the shield member 40 a, 40 b is not limitedif the aperture shape of the pupil of the objective lens can effectivelybe controlled. For example, as shown in FIGS. 20A and 20B, the shieldmembers may be so constructed as to be rotatable about axes 40 p and 40q extending in the right-and-left direction. In this case, too, theaperture shape of the pupil of each objective lens can be effectivelycontrolled.

In the description with reference to FIGS. 17 to 20, the ordinarymicroscopes are employed by way of example. The optical system shown ineach figure is similarly applicable to stereoscopic microscopes. Thepositional relationship between the pupil and shield members in thestereomicroscope is the same as shown in FIG. 11. Specifically, it is asshown in FIGS. 21 to 26B.

FIGS. 27A to 29C show modifications of the shield members. Thesemodifications are suited to ordinary microscopes.

In a structure as shown in FIGS. 27A to 27E, one shield member 40 d hasan L-shape and the other shield member 40 c has a rectangular shape. Theshield members 40 c and 40 d with these shapes are so constructed as tobe movable in the front-and-rear direction and the right-and-leftdirection. In addition, the shield member 40 c is so constructed as tobe rotatable in a plane perpendicular to the optical axis. Thereby, asshown in FIGS. 27A to 27D, the aperture shape of the pupil of theobjective lens, and accordingly the illumination light directly enteringthe objective lens, can be successively adjusted (in the figures, ahatched region in the pupil C is a region shielded by the shield members40 c, 40 d). Furthermore, as shown in FIG. 27E, dark-field illuminationcan be performed by shielding the pupil C alone. In this case, by partlyshielding the pupil A, the light amount at the time of dark-fieldillumination can be successively adjusted and the degree of freedom forobservation of the phase-sample can be improved.

FIGS. 28A and 28B show an example of the structure wherein four squareshield members 40 e are disposed at (or near) positions conjugate withthe pupil position of the objective lens. As is shown in FIGS. 28A and28B, the shield members are moved in the front-and-rear direction andthe right-and-left direction. Thereby, bright-field illumination can beswitched to oblique illumination and to dark-field illumination, whilethe angle of incidence of illumination light is being varied. If eachshield member 40 e is made rotatable or an aperture stop is providedadditionally, the degree of freedom for illumination increases and thedegree of freedom for observation of the phase-sample improves.

FIGS. 29A to 29C show an example of the structure wherein rectangularshield members 40 f each having a quadrantal notch 40 h, as shown inFIG. 29A, are disposed at (or near) a position conjugate with the pupilposition of the objective lens. If the shield members are put incontact, as shown in FIG. 29B, a shield member with a central circularaperture is formed. Using this, oblique illumination can be performedwith a circular aperture. If each shield member is shifted in directionsof arrows, various aperture shapes can be realized, for example, asshown in FIG. 29C.

As has been described above, even where the number of shield members istwo or more or the shape of each shield member is not rectangular, theaperture shape of the pupil of the objective lens can be freelycontrolled. It is possible to visualize a transparent phase-sample andto successively control the contrast.

In the above structure, it is preferable to shift the shield members soas to meet the following condition for the ratio (D2/D1) of D2 to D1:

D 2/D 1≦0.5  (Condition 1)

where D1 is the area of the pupil of the objective lens, and D2 is thearea of the aperture created in the pupil of the objective lens by theshield members. By meeting Condition 1, the transparent phase-sample canbe made visible with good contrast.

Specifically, the ratio (D2/D1) of D2 to D1 is a ratio between directlight directly entering an objective lens through a sample anddiffraction light due to the sample, and it is a numerical valuerepresenting the degree of oblique illumination. If the above conditionis satisfied, diffraction light from the sample can be taken in andoblique illumination with a reduced ratio of direct light or dark-fieldillumination can be realized. Thus, the transparent sample can be madevisible with high contrast.

In the above structure, in order to visualize the transparentphase-sample with high contrast, an illumination optical system having acondenser lens which can take a large illumination angle, as in obliqueillumination or dark-field illumination, is required. Moreover, it isimportant to visualize a phase-sample, as the illumination range of thecondenser lens increases, that is, as the magnification decreases. Ifthe range for observation increases, the efficiency for observationimproves.

In general, a certain relationship is present between the magnificationof an objective lens and the numerical aperture. The magnification andnumeral aperture of the objective lens have the following values inapproximation (TABLE 1):

Magnification of Numerical Objective Lens Aperture   0.5 0.02 1-1.250.04  2 0.08  4 0.16 10 0.4 20 0.7 (Dry System) 0.8 (Oil-ImmersionSystem) 40 0.95 (Dry System) 1.3 (Oil-Immersion System) 100  0.95 (DrySystem) 1.4 (Oil-Immersion System)

Assuming that the maximum numerical aperture at which the condenser lenscan perform illumination is NA1 and the numerical aperture of theobjective lens at which the maximum illumination range of the condenserlens can be observed is NA2, the transmission-illumination opticalsystem should preferably have a condenser lens which satisfies:

NA 2 /NA1<0.6  (Condition 2)

As regards the objective lens with which the maximum illumination rangecan be observed, if Condition 2 is satisfied, sufficient illuminationlight, which has a large illumination angle and does not directly enterthe objective lens, is obtained. Since illumination light components foroblique illumination including dark-field illumination can besufficiently maintained, the degree of freedom for illumination isincreased in a range from oblique illumination to dark-fieldillumination by shifting two or more shield members. As a result, fromobservation with a low-magnification objective lens, the contrast of thetransparent phase-sample can be successively varied. Since the numericalaperture increases as the magnification of the objective lens increases,the region for oblique illumination including dark-field illumination isdecreased in accordance with an increase in magnification of theobjective lens. It is therefore necessary to meet Condition 2, not onlyin low-magnification observation but also in observation with othermagnification.

The shield members are situated at or near the front-side focal positionof the condenser lens, as shown in FIG. 7. However, depending on thedesign of the optical system of the microscope, they may be situated atvarious positions. An example of such an optical system will now bedescribed with reference to FIG. 30.

FIG. 30 shows a structure relating to a modified design of thetransmission-illumination optical system in the optical system shown inFIG. 7 (the same elements as in FIG. 7 are denoted by like referencenumerals). In this transmission-illumination optical system, relaylenses 60 and 61 are provided between the diffusion plate 22 anddeflection mirror 24 shown in FIG. 7. According to this structure, lightfrom the light source 20 is converted to parallel light by means of thecollector lens 21 and then passed through the relay lens 60, therebyforming a primary light-source image (the focal position indicated bysymbol P3). The primary light-source image is projected at thefront-side focal position (a secondary light-source image) via the relaylens 61, deflection mirror 24 and projection lens 25. The stop 23 is afield stop, and the stop 41 functions as an aperture stop.

According to this optical system, the shield members 40 a and 40 b withthe above-described structure (or shield members with some otherstructure described above) can be disposed at or near the position P3 ofthe primary light-source image which is conjugate with the pupil of theobjective lens 31. With this structure, too, each shield member is movedso as to meet Condition 1 and thus the effect of oblique illumination ordark-field illumination is obtained. In addition, in the case ofilluminating a region with a low magnification or a very lowmagnification, the condenser lens is removed from the illuminationoptical path or the condenser lens for use in illumination with amagnification of ×1 or less is constituted as an afocal system. In thiscase, the position conjugate with the pupil position of the objectivelens 31 corresponds to the field stop 23 at the front-side focalposition of the projection lens 25. Accordingly, the same effect isobtained by movably disposing the shield members with the abovestructure at or near the position of the field stop 23.

In the above-described transmission-illumination optical system, thedeflection mirror 24 may be constructed to be rotatable. In the case ofthe stereomicroscope shown in FIG. 6, the deflection mirror 24 isrotated by operating the mirror-inclination adjustment lever 204. Sincethe deflection mirror 24 is thus rotatable, the angle of illuminationlight on the sample can be desirably adjusted in the obliqueillumination mode or dark-field illumination mode.

In the above structure, the condenser lens used in thetransmission-illumination optical system is constructed to be switchableaccording to the magnification of the objective lens. Specifically, atleast one lens group provided between the aperture stop of the condenserlens and the sample is held/detached or replaced with another lens groupin accordance with the objective lens with a low magnification or theobjective lens with a high magnification. In the condenser lens withthis structure, it is preferable to dispose the above-described shieldmembers at or near the pupil position of the condenser lens in thelow-magnification mode.

The reason why the shield members are disposed at or near the pupilposition of the condenser lens in the low-magnification mode will bestated below. With the illumination method according to the presentinvention, exact information is not necessarily be obtained for thesample observed in the high-magnification mode with a large numericalaperture. In the low-magnification mode, a scattering phenomenon due toillumination, rather than a diffraction phenomenon, will occur. Inaddition, in the observation in the low-magnification mode, it isnecessary to make visualization with the contract varied successively,rather than to place much importance on the resolution.

If the shield members for controlling the aperture created in the pupilof the objective lens is disposed at the pupil position of the condenserlens in the low-magnification mode, the illumination can be changedsuccessively from the bright-field illumination to oblique illuminationand then to dark-field illumination, as described above, even in thelow-magnification mode for visualizing the phase-sample. Thus, thecontrast of the phase-sample can be successively varied. In addition, inthe high magnification mode, by using in combination a universalcondenser which is a condenser lens capable of phase-contrastobservation or differential-interference observation, the respectiveillumination modes can be switched. Specifically, a structure can beadopted such that in the observation with a low magnification thestructure and distribution of the entire phase-sample can be madevisible with high contrast using the above-mentioned illumination methodand in the observation with a high magnification the detailed structurecan be observed using the conventional observation method such as thephase-contrast method and differential-interference method.

In a case where the condenser lens wherein, as described above, at leastone lens group provided between the aperture stop of the condenser lensand the sample is held/detached or replaced with another lens group inaccordance with the objective lens with a low magnification or theobjective lens with a high magnification is used in thetransmission-illumination optical system, it is preferable to meet thefollowing condition:

F 1/F 2<0.45  (Condition 3)

where F1 is the focal distance of the condenser lens in thehigh-magnification mode, and F2 is the focal distance of the condenserlens in the low-magnification mode.

If the condenser lens is designed to meet Condition 3, good illuminationcan be achieved by switching the two condenser lenses in a range from ahigh magnification to a low magnification and to a very lowmagnification. In particular, in the range from a low magnification to avery low magnification, the illumination can be freely varied by meansof the above-mentioned shield members for controlling the aperture shapecreated in the pupil of the objective lens. Thus, the transparent phasesample can be visualized with high contrast.

Examples of the structure of the condenser lens used in the microscopetransmission-illumination apparatus of the present invention will now bespecifically described.

Example 1 of Structure

FIG. 31 shows a condenser lens used for a high magnification. A lenssystem comprises lens groups L1, L2 and L3. The condenser lens includesan aperture stop 70 and a special observation turret disc (not shown),such as a phase-contrast ring slit, a differential-interference prism ora dark-field ring slit, provided at a pupil position P1 of the condenserlens. A sample is placed on a slide glass 72 and disposed in a stageplane. The shield members 74 a and 74 b having the above-describedstructure are movably disposed near the pupil position P1.

The structure of the condenser lens will be shown below.

Symbols r1, r2, . . . denote the radii of curvature of respective lensesarranged in order from the light source side.

Symbols d1, d2, . . . denote thickness of air gap or glass member.

Symbols nd1, nd2, . . . denote d-line refractive indices of respectivelenses.

Symbols ν1, ν2, . . . denote Abbe numbers of respective lenses.

The aperture stop (70) is situated at a position of 10.4 from a firstplane on the light-source side.

The pupil (P1) is situated at a position of 5.40 from the first plane onthe light-source side.

r1 = 111.02 d1 = 8.77 nd1 = 1.48749 ν1 = 70.2 r2 = −17.26 d2 = 3.16 nd2= 1.58921 ν2 = 41.1 r3 = −59.01 d3 = 0.11 r4 = 21.59 d4 = 8.69 nd3 =1.741 ν3 = 52.7 r5 = −27.78 d5 = 2.3 nd4 = 1.84666 ν4 = 23.8 r6 = 79.83d6 = 0.23 r7 = 8.28 d7 = 6.9 nd5 = 1.741 ν5 = 52.7 r8 = 12.58 d8 = 3.64r9 = ∞ (stage plane)

The focal distance of the condenser lens is 13.28 mm.

The maximum illuminable numerical aperture NA1 is 0.9.

The magnification of the objective lens corresponding to the maximumilluminable range is ×10.

The numerical aperture NA2 of the ×10-magnification lens is 0.4 fromTable 1.

Since NA2/NA1=0.444, Condition 2 is satisfied.

According to the above condenser lens, the pupil diameter of thecondenser lens is sufficiently greater than that of thelow-magnification objective lens, and oblique-illumination componentsincluding dark-field illumination for illuminating the sample isobtained. Thus, the transparent phase-sample, etc. can be visualized andthe contrast varied successively by movably disposing, near the aperturestop position, the shield members for controlling the shape of theaperture created in the pupil of the objective lens. In addition to theillumination apparatus of the present invention, according to thecondenser lens, the phase-contrast observation,differential-interference observation and dark-field observation can beperformed, and the illumination optical system permitting variousobservation methods is realized. The advantages can be obtained with thestructure wherein the shield members 74 a and 74 b are disposed in theturret disc and moved.

Example 2 of Structure

FIG. 32 shows a condenser lens used for a high magnification. A lenssystem comprises lens groups L1, L2 and L3. The condenser lens includesan aperture stop 70 and a special observation turret disc (not shown),such as a phase-contrast ring slit, a differential-interference prism ora dark-field ring slit, provided at a pupil position P1 of the condenserlens. A sample is placed on a slide glass 72 and disposed in a stageplane. In this case, oil for oil-immersion is filled between the lens L3and the stage. The shield members 74 a and 74 b having theabove-described structure are movably disposed near the pupil positionP1.

The structure of the condenser lens will be shown below.

Symbols r1, r2, . . . denote the radii of curvature of respective lensesarranged in order from the light source side.

Symbols d1, d2, . . . denote thickness of air gap or glass member.

Symbols nd1, nd2, . . . denote d-line refractive indices of respectivelenses.

Symbols ν1, ν2, . . . denote Abbe numbers of respective lenses.

The aperture stop (70) is situated at a position of 5.25 from a firstplane on the light-source side.

The pupil (P1) is situated at a position of 0.25 from the first plane onthe light-source side.

r1 = 43.66 d1 = 9.0 nd1 = 1.56873 ν1 = 63.2 r2 = −21.71 d2 = 1.9 nd2 =1.78472 ν2 = 25.7 r3 = −61.47 d3 = 0.3 r4 = 12.79 d4 = 6.4 nd3 = 1.58913ν3 = 61.0 r5 = 24.9 d5 = 0.3 r6 = 7.07 d6 = 8.0 nd4 = 1.62041 ν4 = 60.3r7 = −34.01 d7 = 1.1 nd5 = 1.72825 ν5 = 28.5 r8 = ∞ d8 = 0.6 (nd6 =1.515 ν6 = 43.1) r9 = ∞ (stage plane)

The focal distance of the condenser lens is 10.00 mm.

The maximum illuminable numerical aperture NA1 is 1.37.

The magnification of the objective lens corresponding to the maximumilluminable range is ×20.

The numerical aperture NA2 of the ×20-magnification lens is 0.7 fromTable 1.

Since NA2/NA1=0.511, Condition 2 is satisfied.

According to the above condenser lens, the pupil diameter of thecondenser lens is sufficiently greater than that of thelow-magnification objective lens, and oblique-illumination componentsincluding dark-field illumination for illuminating the sample isobtained. Thus, the transparent phase-sample, etc. can be visualized andthe contrast varied successively by movably disposing, near the aperturestop position, the shield members for controlling the shape of theaperture created in the pupil of the objective lens. In addition to theillumination apparatus of the present invention, according to thecondenser lens, the phase-contrast observation,differential-interference observation and dark-field observation can beperformed, and the illumination optical system permitting variousobservation methods is realized. The advantages can be obtained with thestructure wherein the shield members 74 a and 74 b are disposed in theturret disc and moved.

Example 3 of Structure

FIG. 33 shows a condenser lens used for a low magnification. Shieldmembers 74 a and 74 b are movably provided for controlling the shape ofthe aperture created in the objective lens. This condenser lens has, ina lens system, an aperture stop 70 and five lenses including junction. Asample is placed on a slide glass 72 and disposed in a stage plane. Theshield members 74 a and 74 b are disposed near the aperture stop 70which is close to the pupil position.

The structure of the condenser lens will be shown below.

Symbols r1, r2, . . . denote the radii of curvature of respective lensesarranged in order from the light source side.

Symbols d1, d2, . . . denote thickness of air gap or glass member.

Symbols nd1, nd2, . . . denote d-line refractive indices of respectivelenses.

Symbols ν1, ν2, . . . denote Abbe numbers of respective lenses.

The aperture stop (70) and pupil (P1) are situated at a position of 20.0from a fourth plane on the sample side.

r1 = 27.892 d1 = 5.3 nd1 = 1.77250 ν1 = 49.6 r2 = ∞ d2 = 2.34 r3 =−92.482 d3 = 2.80 nd2 = 1.74077 ν2 = 27.79 r4 = 92.482 d4 = 48.91 r5 =−19.919 d5 = 2.45 nd3 = 1.84666 ν3 = 23.78 r6 = ∞ d6 = 5.55 nd4 =1.59551 ν4 = 39.21 r7 = −18.184 d7 = 0.2 r8 = ∞ d8 = 3.59 nd5 = 1.7725ν5 = 49.6 r9 = −34.61 d9 = 3.8 r9 = ∞ (stage plane)

The focal distance of the condenser lens is 74.94 mm.

The maximum illuminable numerical aperture NA1 is 0.16.

The magnification of the objective lens corresponding to the maximumilluminable range is ×1.25.

The numerical aperture NA2 of the ×20-magnification lens is 0.04 fromTable 1.

Since NA2/NA1=0.25, Condition 2 is satisfied.

According to the above condenser lens, the pupil diameter of thecondenser lens is sufficiently greater than that of thevery-low-magnification objective lens, and oblique-illuminationcomponents including dark-field illumination for illuminating the sampleis obtained. Thus, the transparent phase-sample, etc. can be visualizedand the contrast varied successively by movably disposing, near theaperture stop position, the shield members for controlling the shape ofthe aperture created in the pupil of the objective lens. In addition, asdescribed in connection with the prior art, in the region with thismagnification there is no illumination for visualizing the phase-sampleand varying the contrast. According to this example of structure, theillumination which is unavailable in the prior art can be realized.

Example 4 of Structure

FIGS. 34A and 34B show the structure of a condenser lens in which atleast one lens group provided between an aperture stop and a sample canbe switched according to a high magnification and a low magnification.FIG. 34A shows a structure for a high-magnification mode and FIG. 34Bshows a structure for a low-magnification mode.

The condenser lens used in the high-magnification mode has the samestructure as shown in FIG. 31. In the low-magnification mode, lensgroups L2 and L3 are shifted out of the illumination optical path and alens group L4, in turn, is shifted into the illumination optical path.Shield members 74 a and 74 b for controlling the aperture created inpupil of the objective lens are movably disposed near the pupil positionP1 in the low-magnification mode.

The range of illumination in the high-magnification mode corresponds toa range of ×10 to ×100, and the range of illumination in thelow-magnification mode corresponds to a range of ×1.25 to ×4.

The structure of the condenser lens will be shown below.

Symbols r1, r2, . . . denote the radii of curvature of respective lensesarranged in order from the light source side.

Symbols d1, d2, . . . denote thickness of air gap or glass member.

Symbols nd1, nd2, . . . denote d-line refractive indices of respectivelenses.

Symbols ν1, ν2, . . . denote Abbe numbers of respective lenses.

(In the high-magnification mode: ×10 to ×100)

The aperture stop (70) is situated at a position of 10.4 from a firstplane on the light-source side.

The pupil (P1) is situated at a position of 5.40 from the first plane onthe light-source side.

r1 = 111.02 d1 = 8.77 nd1 = 1.48749 ν1 = 70.2 r2 = −17.26 d2 = 3.16 nd2= 1.58921 ν2 = 41.1 r3 = −59.01 d3 = 0.11 r4 = 21.59 d4 = 8.69 nd3 =1.741 ν3 = 52.7 r5 = −27.78 d5 = 2.3 nd4 = 1.84666 ν4 = 23.8 r6 = 79.83d6 = 0.23 r7 = 8.28 d7 = 6.9 nd5 = 1.741 ν5 = 52.7 r8 = 12.58 d8 = 3.64r9 = ∞ (stage plane)

(In the low-magnification mode: ×1.25 to ×4)

The aperture stop (70) is situated at a position of 10.4 from the firstplane on the light-source side.

The pupil (P1) is situated at a position of 24.3 from the first plane onthe light-source side.

r1 = 111.02 d1 = 8.77 nd1 = 1.48749 ν1 = 70.2 r2 = −17.26 d2 = 3.16 nd2= 1.58921 ν2 = 41.1 r3 = −59.01 d3 = 11.33 r4 = ∞ d4 = 6.9 nd3 = 1.51633ν3 = 64.1 r5 = −30.0 d5 = 3.64

The focal distance F1 of the condenser lens in the high-magnificationmode is 13.29.

The focal distance F2 of the condenser lens in the low-magnificationmode is 43.63.

F 1/F 2=0.30

According to the above condenser lens, the shield members are disposednear the pupil position in the low-magnification mode. Thus, in therange of ×1.25 to ×4, the phase-sample can be visualized and thecontrast varied successively. Accordingly, in the region of a very lowmagnification to a low magnification, the phase-sample can be visualizedand observed using the above-described low-magnification condenser lens.In addition, in the high-magnification mode, the phase-contrastobservation, differential-interference observation and dark-fieldobservation can be performed.

Example 5 of Structure

FIGS. 35A and 35B show a structure of a condenser lens which is switchedin use in accordance with a high magnification and a low magnification.FIG. 35A shows a structure for use in the high-magnification mode, andFIG. 35B shows a structure for use in the low-magnification mode. Inthis case, the condenser lens used in the high-magnification mode hasthe same structure as shown in FIG. 31, and the condenser lens used inthe low-magnification mode has the same structure as shown in FIG. 33.

The structure of the condenser lens will be shown below.

Symbols r1, r2, . . . denote the radii of curvature of respective lensesarranged in order from the light source side.

Symbols d1, d2, . . . denote thickness of air gap or glass member.

Symbols nd1, nd2, . . . denote d-line refractive indices of respectivelenses.

Symbols ν1, ν2, . . . denote Abbe numbers of respective lenses.

(In the high-magnification mode: ×10 to ×100)

The aperture stop (70) is situated at a position of 10.4 from a firstplane on the light-source side.

The pupil (P1) is situated at a position of 5.40 from the first plane onthe light-source side.

r1 = 111.02 d1 = 8.77 nd1 = 1.48749 ν1 = 70.2 r2 = −17.26 d2 = 3.16 nd2= 1.58921 ν2 = 41.1 r3 = −59.01 d3 = 0.11 r4 = 21.59 d4 = 8.69 nd3 =1.741 ν3 = 52.7 r5 = −27.78 d5 = 2.3 nd4 = 1.84666 ν4 = 23.8 r6 = 79.83d6 = 0.23 r7 = 8.28 d7 = 6.9 nd5 = 1.741 ν5 = 52.7 r8 = 12.58 d8 = 3.64r9 = ∞ (stage plane)

(In the low-magnification mode: ×1.25 to ×4)

The aperture stop (70) and pupil (P1) are situated at a position of 20.0from a fourth plane on the sample side.

r1 = 27.892 d1 = 5.3 nd1 = 1.77250 ν1 = 49.6 r2 = ∞ d2 = 2.34 r3 =−92.482 d3 = 2.80 nd2 = 1.74077 ν2 = 27.79 r4 = 92.482 d4 = 48.91 r5 =−19.919 d5 = 2.45 nd3 = 1.84666 ν3 = 23.78 r6 = ∞ d6 = 5.55 nd4 =1.59551 ν4 = 39.21 r7 = −18.184 d7 = 0.2 r8 = ∞ d8 = 3.59 nd5 = 1.7725ν= 49.6 r9 = −34.61 d9 = 3.8 r10 = ∞ (stage plane)

The focal distance of the condenser lens is 74.94 mm.

The focal distance F1 of the condenser lens in the high-magnificationmode is 13.29.

The focal distance F2 of the condenser lens in the low-magnificationmode is 74.94.

F 1/F 2=0.18

According to the above condenser lens, the shield members are disposednear the pupil position in the low-magnification mode. Thus, in therange of ×1.25 to ×4, the phase-sample can be visualized and thecontrast varied successively. As regards the condenser lens in thehigh-magnification mode, since the optical element for phase-contrastobservation and differential-interference observation is disposed at thepupil position of the condenser lens, such observation can be performed.

Accordingly, in the region of a very low magnification to a lowmagnification, the phase-sample can be visualized and observed using theabove-described low-magnification condenser lens. In addition, in thehigh-magnification mode, the phase-contrast observation,differential-interference observation and dark-field observation can beperformed, and the illumination optical system permitting variousobservation methods is realized. Since the positions of the shieldmembers 74 a, 74 b situated in the low-magnification mode are close tothe pupil position of the condenser lens in the high-magnification mode,the aperture created in the pupil of the objective lens can becontrolled using the shield members 74 a and 74 b even with thecondenser lens in the high-magnification mode.

The above-described transmission-illumination optical system may becombined in use with a reflected-illumination fluorescent microscope(not shown). Since there is no need to dispose the phase-film at thepupil position of the objective lens, unlike the phase-contrastobservation objective lens, no loss occurs in the objective lens andfluorescence can be observed with high brightness. Afluorescence-colored transparent phase-sample can be visualized withhigh contrast by using the above-described transmission-illuminationsystem, whereby a cell, etc. which is fluorescence-colored byreflected-fluorescence-illumination, can be observed.

A second embodiment of the present invention will now be described withreference to FIG. 36. FIG. 36 shows a schematic structure of atransmission-illumination optical system in a stereomicroscope.

The transmission-illumination optical system comprises a parallel-beammember 82, such as a collector lens, for converting light from a lightsource 80 such as a halogen lamp; a first diffusion plate 83, such as afrosted glass, for diffusing a light beam from the parallel-beam member82; a first collection member 85, such as a convex lens, for collectingdiffused light from the first diffusion plate 83; a second diffusionplate 86, such as a frosted glass, for diffusing light from the firstcollection member 85; a deflection mirror 87 for deflecting upward thelight from the second diffusion plate 86; and a second collection member88, such as a convex lens, for collecting light from the deflectionmirror 87 and radiating the light onto a sample 90 on a sample-mountingglass 89.

A first auxiliary convex lens 91 is disposed between the seconddiffusion plate 86 and deflection mirror 87 so as to be shiftable fromthe optical axis. A second auxiliary convex lens 92 is disposed betweenthe mirror 87 and the second collection member 88 so as to be shiftable.First and second shield members 95 a and 95 b having the same structureas the shield members of the preceding embodiment are movably disposedbetween the second collection member 88 and second auxiliary convex lens92. Shield members 95 c and 95 d having the same structure are movablydisposed between the second diffusion member 86 and first auxiliaryconvex lens 91.

According to the above structure, light from the light source 80 isefficiently collected and substantial paralleled by the parallel-beammember 82 and made incident on the first diffusion plate 83. The firstdiffusion plate 83 functions as a substantially uniform, large-arealight source for the illumination field. The light diffused by the firstdiffusion plate 83 is collected by the first collection member 85. Thefirst collection member 85 functions to collect the light, which hasbeen diffused by the first diffusion plate 83 in scattering directions,in a converging direction effective for illumination.

The light incident on the second diffusion plate 86 is further diffusedalong the converging direction. The second diffusion plate 86 diffuseslight to meet the numerical aperture and becomes a final light source.The light diffused by the second diffusion plate 86 is deflected upwardby the deflection mirror 87, made incident on the second collectionmember 88, and passed through the sample-mounting transparent member 89,thus illuminating the sample 90.

The first auxiliary convex lens 91 interposed between the seconddiffusion plate 86 and deflection mirror 87 functions to intensity thelight beam and enhance the efficiency of use of light for thehigh-magnification objective lens having a narrower illumination fieldand a greater numerical aperture. The second auxiliary convex lens 92interposed between the deflection mirror 87 and second collection member88 functions to enhance the power of the convex lens in combination withthe second collection member 88, thereby narrowing the illuminationfield and illuminating the sample 90 at a greater angle. In other words,since the illumination optical system is switched according to themagnification of the objective lens, observation can be made under theoptimal illumination condition.

The high-magnification objective lens has a shorter focal distance, andthe pupil-conjugate position in the illumination apparatus becomes asclose as possible to the second collection member 88. In the case of thelow-magnification objective lens, the pupil-conjugate position normallybecomes away from there and located before the turning point of theoptical axis at the deflection mirror 87. Accordingly, the brightnessreduction is made by independently shifting each shield member 95 a, 95b, and 95 c, 95 d to/from the optical axis, and also the obliqueillumination can be achieved by displacing, as desired, each shieldmember from the optical axis.

The shield members 95 a, 95 b (95 c, 95 d) can be shifted by the drivingmechanisms shown in FIGS. 13 to 16B. The pupils of the right and leftobjective lenses are uniformly reduced by the shield members in theup-and-down direction, as shown in FIGS. 12B to 12D and FIGS. 21 to 26B.Since the right and left pupils are uniformly reduced, the right andleft images are uniformly viewed. Thus, a natural stereoscopic effect iscreated by the right-and-left parallax characterizing thestereomicroscope. Moreover, like the above-described embodiment, theratio between direct light incident on the object lens of pupil anddiffraction light can be controlled by shifting the shield members.Accordingly, the contrast can be increased or varied successively.Specifically, very fine contrast adjustment can be made for a samplewith a fine structure. A matter, which is not observable in the priorart, can be observed. Since the stop is disposed at the positionsuitable for high magnification and low magnification, obliqueillumination can be performed in a range from high magnification to lowmagnification. Furthermore, since the switching between the lowmagnification and high magnification is achieved by the addition of thelenses 91 and 92, the structure is simple and inexpensive. Since the twodiffusion plates are disposed and the function of each diffusion plateis made clear, the optimal design for the optical system can easily bemade with higher efficiency. There is no need to use a diffusion platewhich has an unnecessarily great diffusion effect.

FIG. 37 shows a modification of the structure shown in FIG. 36. Thismodification differs from the structure shown in FIG. 36 in that thedeflection mirror 87 is made rotatable and the low-magnification-sideshield members 95 c and 95 d are removed.

This is based on a demand for high-magnification observation in a caseof observing a fine structure, mainly from the standpoint of resolution.As regards low-magnification observation, a large visual field with asufficient illumination effect (oblique illumination) can be obtainedwith such a contrast as in the prior art by means of the rotatabledeflection mirror 87 a. In this way, high-magnification obliqueillumination is performed using the shield members 95 a and 95 b, whilelow-magnification oblique illumination is performed using the deflectionmirror 87 a. Thus, the cost can be reduced. Besides, the operability ishigh since the oblique illumination can be made with the deflectionmirror 87 a at a middle magnification at which the conjugaterelationship of the pupil position is not sufficient.

The illumination system according to the embodiment shown in FIGS. 36and 37 is an example wherein the requirements for the illumination fieldand numerical aperture (pupil) are met at high magnification and lowmagnification. Accordingly, even if the above-described shield membersare disposed at the pupil position of the conventional bright-fieldillumination apparatus, oblique illumination can be performedadequately. However, in order to fully exhibit general-purposeproperties and effect in the oblique illumination, it is preferable tocombine the illumination system with the above-described illuminationsystem or an optical system having a wider visual field and a largernumerical aperture.

In the illumination optical system shown in the figure, it is possibleto integrate the first and second diffusion plates 83 and 86, dispensewith the first collection member 85, and have the lens effect shared bythe first and second diffusion plates. It is also possible to vary thefocal distances of the first and second collection members 85 and 88,instead of interposing the first and second auxiliary convex lenses 91and 92. Moreover, it is possible to change the positions where theauxiliary convex lenses are interposed.

In the embodiments shown in FIGS. 36 and 37, the following modificationsmay be made.

At least two shield members may be movably disposed at two locationsconjugate with the high-magnification pupil position andlow-magnification pupil position of a zoom-stereomicroscope. With thisstructure, optimal oblique illumination can be achieved at highmagnification and low magnification.

The optical system shown in the figure may be provided with anotherdeflection member for deflecting upward the light-emission optical axisfrom the light source. This deflection member is inclined to inclineillumination light. In combination with the inclination of anotherdeflection member, the range of oblique illumination is increased.

A third embodiment of the present invention will now be described.

FIG. 38A is a side view showing a transmission-illumination opticalapparatus disposed in a transmission-illumination frame of astereomicroscope. The stereomicroscope has the structure as shown inFIG. 6, and so the shape of the entirety thereof is not shown.

In the transmission-illumination optical apparatus according to thisembodiment, a bright-field optical system and a dark-field opticalsystem are switchable by means of an optical system switching mechanism(to be described later). FIG. 38A shows the bright-field optical system.

A light source 401 such as a halogen lamp is provided within a housing300. Light from the light source 401 is converted to a substantialparallel beam through a collector lens 402. The substantial parallelbeam is deflected by a deflection member (deflection mirror) 406 toilluminate a sample 309 placed on a sample-mounting glass(specimen-mounting glass) 408 provided in an opening 301 a formed in anupper surface 301 of the housing 300. In this case, the collector lens402 is disposed such that the light-emission optical axis of the lightsource 401 is inclined in an oblique downward direction by about 5 to 10degrees (6 degrees in this embodiment) to the horizontal direction.

On the optical axis between the light source 401 and deflection member406, there are provided filters 410, 411 and 412 which can be shiftedto/from the optical axis by means of a filter shifting mechanism (to bedescribed later), a diffusion plate 415, and a convex lens 420 of asubstantially oval shape (see FIG. 38B) obtained by cutting out upperand lower portions of a circular lens. In addition, a convex lens 407having a Fresnel surface 407 a and a diffusion surface 407 b is providedbetween the deflection member 406 and sample-mounting glass 408.

In the above structure, the switchable elements for the bright-fieldoptical system are the deflection member 406, convex lens 407 anddiffusion plate 415.

FIG. 39A shows the dark-field optical system. The dark-field opticalsystem includes a second deflection member 430 for deflecting upward thelight emitted from the light source 401, and a shield member 435 forobliquely radiating the deflected light to the sample 309. In order toreflect the light reflected by the second deflection member toward thecircumference from the center at the optical axis, the shield member 435comprises an upwardly opening conical reflection mirror 436; a circularshield plate 437 attached to a bottom portion of the reflection mirror436; and a cylindrical reflection mirror 438 for making the lightreflected from the reflection mirror 436 incident obliquely on thesample 309. With this structure, the light deflected upward by thesecond deflection member 430 is shut off by the circular shield plate437. The reflection mirrors 436 and 438 produce annular illuminationwith a large opening angle, thus subjecting the sample 309 to dark-fieldillumination via the sample-mounting glass 408.

The second deflection member 430 and shield member 435 may be integrallyformed of, e.g. a resin. In this case, the above-mentioned mirror may beattached to a light reflection portion, or aluminum for reflecting lightmay be deposited thereon.

In the above structure, the switchable elements for the dark-fieldoptical system are the second deflection member 430 and shield member435.

Referring to FIGS. 38A, 39A and 40, a description will now be given ofthe optical system switching mechanism for switching the bright-fieldoptical system and dark-field optical system. FIG. 40 shows the opticalsystem switching mechanism, as viewed in a direction A in FIGS. 38A and39A, with a bottom plate 302 of the housing removed. In the figures,numerals 440 and 450 denote the bright-field optical system anddark-field optical system.

The bright-field optical system 440 comprises the diffusion member 415,deflection member 406 and convex lens 407, as described above, and theseare integrally coupled, at their proximal end portions, to abright-field-side support member 460 having an annular mounting portion460 a. The dark-field optical system 450 comprises the second deflectionmember 430 and shield member 435, as described above, and these areintegrally coupled, at their proximal end portions, to a dark-field-sidesupport member 463 having an annular mounting portion 463 a.

A cylindrical shaft 470 is vertically fixed to the upper surface 301 ofthe housing 300 at a position opposite to the light source 401. Themounting portion 460 a of bright-field-side support member 460 and themounting portion 463 a of dark-field-side support member 463 arerotatably fitted on the shaft 470. In addition, an annular mountingportion 480 a formed at the proximal end of an operation lever 480 isrotatably provided on the shaft 470. The mounting portions 460 a and 463a are fixed to the mounting portion 480 a by means of a fixing section(not shown). A knob portion 480 b is formed at a distal end portion ofthe lever 480. The knob portion is projected from a lever operation slot303 b formed in a side surface of the housing 300.

Accordingly, by shifting the knob portion 480 b of lever 480 along theslot 303 b, a switching operation can be performed so that either thebright-field optical system 440 or the dark-field optical system 450 maybe situated on the optical axis (FIG. 40 showing a state in which thedark-field optical system is situated on the optical axis).

By referring to FIGS. 38A and 41, the mechanism for shifting the filters410, 411 and 412 will now be described. Each filter is horizontallyshifted off the optical axis.

FIG. 41 is a view taken in a direction B in FIG. 38A, with the bottomplate 302 of the housing removed. The filters 410, 411 and 412 aredisposed to intersect at right angles with the optical axis (inclined 6degrees downward) which is determined as described above. The respectivefilters are supported at one end on support arms 510, 511 and 512. Thesupport arms are rotatably supported at the other end on three verticalshafts 520, 521 and 522 fixed to the upper surface of the housing 300.

The housing 300 is provided with three operation shafts 530 to 532associated with the support arms 510, 511 and 512. The three operationshafts 530 to 532 can be pushed/pulled by a predetermined stroke. A pinis fixed to one end portion of each operation shaft. (The figure shows apin 532 a of the operation shaft 532 alone). The respective pins arepassed through slots (the figure showing a slot 512 a in support arm 512alone) in the respective support arms 510, 511 and 512. As a result, ifthe operation shaft 532 is pulled out to a position indicated by atwo-dot-and-dash line, the pin 532 a moves along the slot 512 a androtates the support arm 512 about the vertical shaft 522. Thus, thefilter 412 is shifted to a position off the optical axis, as indicatedby a two-dot-and-dash line. On the other hand, if the operation shaft532 is returned from that position to a position indicated by a solidline, the filter 412 is shifted to the position on the optical axis, asindicated by the solid line. The same push/pull operation can beperformed for the other filters 410 and 411 by means of the operationshafts 530 and 531.

According to the transmission-illumination optical apparatus with theabove structure, the optical path length from the light source to thesample is great, and, in particular, the optical path is elongated inthe horizontal direction. Thus, the optical element such as theabove-described filter can be disposed without increasing the heightbetween the upper surface of the stage, on which the sample is mounted,and the bottom surface of the housing. In particular, since each filterrotates in the horizontal plane and shifted onto/off the optical axis,the height of the housing is not increased. Specifically, since thefilter 410, 411, 412 is horizontally rotated off the optical axis, theposition thereof in the height direction is unchanged and there is noneed to unnecessarily increase the height of the housing. In addition,since each filter is coupled to the operation shaft 530, 531, 532 bymeans of the above-described link mechanism, a slight operation amount(the amount of a pulling operation for the operation shaft) is adequateto shift each filter onto/off the optical axis.

In the above structure, the diffusion member 415 greatly contributes todetermining the illumination field. If the degree of diffusion isincreased, a wider illumination field is covered. If the degree ofdiffusion is decreased, a narrower illumination field is covered. By theshifting of the diffusion member, the range of field can be controlled.Bright illumination can be performed for a narrower field. Thesubstantially oval convex lens 420, as shown in FIGS. 38B and 39B, hassuch a shape that the upper and lower circumferential portions of acircle are cut out. The reason for this is that when a sample is to beobserved by a stereomicroscope, sufficient illumination needs to beprovided in the right-and-left direction and a lens with a largediameter needs to be used in order to increase the numerical aperture.As regards illumination in the front-and-back direction, a lens withsuch a large diameter is not needed. In other words, even if thediameter of the lens in the up-and-down direction is small, sufficientillumination can be provided in the right-and-left direction. An opticalsystem with a small dimension in the up-and-down direction can beconstructed using the substantially oval convex lens 420 with theirupper and lower circumferential portions cut out. Such a lens can beformed of a resin.

The deflection member 406 vertically deflects the optical axis which isinclined 6 degrees to the horizontal direction. Accordingly, thedeflection member 406 can be disposed such that the angle for lightincidence and light emission is 84 degrees, that is, the incident lightis reflected at 42 degrees to the line normal to the mirror surface, asshown in FIG. 42B. Where the diameter of the required light beam is φ40,the height is 40−40×tan 42°=4 and the height of the apparatus can bethinned by 4 mm, compared to the ordinary case of reflection at 45degrees, as shown in FIG. 42A.

If a diffusion plate is inserted in the filter 410 and it is shiftedin/out along with the filter 410, the degree of diffusion of thediffusion plate 415 in the bright-field optical system can be varied andthe illumination field controlled. If a Fresnel lens is substituted forthe convex lens 407, the thickness of the lens can be thinned even if itis large and the degree of diffusion of the diffusion plate 415 can beincreased to produce scattering light with high intensity. In this case,light is bent by the Fresnel surface 407 a in a converging direction andpassed through the diffusion surface 407 b. Compared to the illuminationfield of the ordinary microscope which is about φ35, an illuminationfield with φ60 to φ70 can be obtained. In other words, about four timesthe area can be illuminated.

On the other hand, in the dark-field optical system, the diffusionmember 415 is shifted off the optical path and the non-diffused light isconverged by the convex lens 420. The converged light is guided to thesecond deflection member 430 and shield member 435, thereby obliquelyilluminating the sample 309. In this case, the circular shield plate 437shuts off leak light from below and darkens the background of the darkfield.

Whichever of the bright-field optical system and the dark-field opticalsystem is switched, either the bright-field optical system or thedark-field optical system can completely use the light taken in by thecollector lens 402. Accordingly, uniform illumination with highefficiency and high brightness can be performed. The illumination pathincluding the dark-field illumination system can be elongated, and awide field can be illuminated with little non-uniformity. Moreover,since the diffusion surface 407 a is provided at the final plane of thebright field, a very large field can be illuminated.

Since the optical axis is inclined at a predetermined angle, the pluralfilters 410, 411 and 412 are disposed along the optical axis with theirheight positions displaced from one another. Accordingly, the supportarms can be disposed along the optical axis in an overlapping manner andthe plural filters can be supported in a compact form.

The shift-in/out mechanism for each filter may be of the rotary type, asshown in FIG. 41, as well as of the slide type, for example.Specifically, as shown in FIGS. 43A and 43B, the filter 410 is held toone end portion of an operation plate 550, and the other end portion ofthe operation plate 550 is projected out of the upper surface 301 of thehousing. This other end portion is held and operated in a range ofstroke L.

According to the above-described transmission-illumination opticalapparatus, the following advantages can be obtained.

(1) Without narrowing the visual field with which the sample can beilluminated, the height of the housing can be reduced, that is, thedimension between the sample-mounting surface and the bottom surface canbe reduced.

(2) When the microscope having the transmission-illumination opticalapparatus is actually designed, the upper surface of the transparentmember, on which the sample is placed, can be made broad and thin.

(3) Since the filters 410 to 412 are built in, the illumination can bealtered without moving the sample 309. Moreover, such filters do notadversely affect the thickness of the apparatus.

(4) In the dark-field illumination system, the light beam emitted fromthe light source 401 can be used without waste and bright illuminationcan be performed. In the bright-field illumination system, the opticalpath can be elongated in design and thus a wide illumination field withless non-uniformity can be obtained with ease. Moreover, since theoptical systems are switched by the single lever 480, a desirableillumination method using the bright-field optical system or dark-fieldoptical system can be easily chosen in accordance with the sample. Inthis case, bright illumination is performed for dark-field illumination,and uniform illumination performed for bright-field observation.

(5) Since each optical system is switched by horizontal rotation, thethickness of the apparatus can be reduced.

(6) By virtue of the adoption of the Fresnel lens 407 and the structureof the convex lens 420, the thickness of the optical apparatus can bereduced.

(7) Where a wide field is to be illuminated, deficiency of peripherallight can be avoided by using the diffusion plate and increasing theangle of diffusion.

A modification of the above-described third embodiment will now bedescribed with reference to FIGS. 44A to 46. In this modification, thesame elements as in the above embodiment are denoted by like referencenumerals and a description thereof is omitted.

The dark-field optical system in this modification has the samestructure as shown in FIG. 39A, and the bright-field optical system isconstructed as shown in FIG. 44A. In this modification, the optical axisis inclined 10 degrees to the horizontal direction.

In the bright-field optical system, as shown in the figure, a seconddiffusion plate 570 is disposed behind the convex lens 420. In addition,the light deflected by the deflection member 406 is collected by aconvex lens 580. The first diffusion plate 415, second diffusion plate570, deflection member 406 and convex lens 580 are constructed to berotated as one body by means of the switching mechanism.

The filter shift-in/out mechanism in this modification is constructed asshown in FIG. 46. The respective filters 410, 411 and 412 are disposedperpendicular to the optical axis inclined by 10 degrees such that theirheight positions differ from one another. The filters are supported atone end to the support arms 510, 511 and 512, and the support arms arerotatably supported at the other end to the three vertical shafts 520,521 and 522 fixed to the upper surface of the housing 300. Rotary knobs730, 731 and 732 are rotatably attached to the housing 300 so as topermit an external rotating operation. The rotary knobs and the verticalshafts 520, 521 and 522 are coupled by means of ring-shaped belts 750.By rotating the rotary knobs, the filters 410, 411 and 412 are switchedto positions indicated by two-dot-and-dash lines or positions indicatedby solid lines.

According to the above structure, the deflection member 406 verticallydeflects the optical axis which is inclined 10 degrees to the horizontaldirection. Accordingly, the deflection member 406 can be disposed suchthat the angle for light incidence and light emission is 80 degrees,that is, the incident light is reflected at 40 degrees to the linenormal to the mirror surface. Where the diameter of the required lightbeam is φ40, the height is 40−40×tan 40°=6.4 and the height of theapparatus can be thinned by 6.4 mm, compared to the ordinary case ofreflection at 45 degrees.

In the above-described structure, the angle of inclination of thelight-emission optical axis from the light source 401 is 6 degrees or 10degrees. According to experimental results, the above operationaladvantages can be obtained if the angle is about 5 to 10 degrees. If theangle of inclination of the light-emission optical axis from the lightsource 401 is too small, the effect of thin size is lost. If the angleof inclination of the light-emission optical axis from the light source401 is too large, the illumination beam is led to a position higher thanthe upper surface of the sample-mounting transparent member and thesample-mounting transparent member is restricted.

In the mirror 436, 438 of the above-described dark-field optical systemmay collect and scatter light at its curved portion other than theconical or cylindrical portion shown in the figures. Each mirror 436,438 may be processed with metal.

Aside from the above-described embodiments, the bright-field opticalsystem may freely be combined with some other optical member. Theswitching of the diffusion member may be performed not by the switchingmechanism of the above-described optical system, but by a structure suchas the above-described filter shift-in/out mechanism such that thediffusion member may be independently shifted onto/off the optical axis.The belts 750 of the filter shift-in/out mechanism shown in FIG. 46 maybe replaced with gears. The Fresnel lens 407 having the integrallyformed diffusion surface may be replaced with the ordinary lens 580, asshown in FIG. 44A if an increase in thickness is permitted. In thiscase, the diffusion plate 570 functions similarly with the diffusionsurface 407 a.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A transmission-illumination apparatus for use ina microscope, the apparatus comprising: a transmission-illuminationoptical system having a light source, a condenser lens for collectinglight emitted from the light source and illuminating a sample, and ashield; and an observation optical system including an objective lensfor observing the sample, the condenser lens including at least one lensgroup constructed to be attachable/detachable or switchable inaccordance with a low magnification and a high magnification of theobjective lens, and the shield being disposed at or near a pupilposition of the condenser lens in a low-magnification mode thereof,which is conjugate with a pupil position of the objective lens, andcontrolling a shape of an aperture formed within a pupil of theobjective lens, wherein the condenser lenses are switched to satisfy acondition, NA2/NA1<0.6, where NA1 is a maximum illuminable numericalaperture of the condenser lens and. NA2 is a numerical aperture of theobjective lens at which a maximum illumination range of the condenserlens is observed.
 2. A microscope transmission-illumination apparatusaccording to claim 1, wherein the pupil position of the condenser lensin a high-magnification mode thereof is located at or near the pupilposition of the condenser lens in the low-magnification mode.
 3. Amicroscope transmission-illumination apparatus according to claim 1,which further comprises an optical member disposed at or near a positionof the shield, for controlling light amount distribution of the apertureformed within the pupil of the objective lens.
 4. A microscopetransmission-illumination apparatus according to claim 3, wherein theshield comprises at least two shield members.
 5. A microscopetransmission-illumination apparatus according to claim 1, which is atransmission-illumination apparatus for a stereoscopic microscope. 6.transmission-illumination apparatus according to claim 1 wherein theshield is shiftable in accordance with a low magnification and a highmagnification of the objective lens, to satisfy a condition, D2/D1≦0.5,where D1 is an area of the pupil of the objective lens and D2 is an areaof an aperture portion created in the pupil of the objective lens.
 7. Atransmission-illumination apparatus for use in a microscope, theapparatus comprising: a transmission-illumination optical system havinga light source, a condenser lens for collecting light emitted from thelight source and illuminating a sample, and a shield; and an observationoptical system including an objective lens for observing the sample, thecondenser lens Including at least one lens group constructed to beattachable/detachable or switchable in accordance with a lowmagnification and a high magnification of the objective lens, and theshield being disposed at or near a pupil position of the condenser lensin a low-magnification mode thereof, which is conjugate with a pupilposition of the objective lens, and controlling a shape of an apertureformed within a pupil of the objective lens, wherein the shield isshiftable in accordance with a low magnification and a highmagnification of the objective lens, to satisfy a condition, D2/D1≦0.5,where D1 is an area of the pupil of the objective lens and D2 is an areaof an aperture portion created in the pupils of the objective lens.
 8. Amicroscope transmission-illumination apparatus according to claim 1,wherein the pupil position of the condenser lens in a high-magnificationmode thereof is located at or near the pupil position of the condenserlens in the low-magnification mode.
 9. A microscopetransmission-illumination apparatus according to claim 7, which furthercomprises an optical member disposed at or near a position of theshield, for controlling light amount distribution of the aperture formedwithin the pupil of the objective lens.
 10. A microscopetransmission-illumination apparatus according to claim 9, wherein theshield comprises at least two shield members.
 11. A microscopetransmission-illumination apparatus according to claim 7, which is atransmission-illumination apparatus for a stereoscopic microscope.
 12. Atransmission-illumination apparatus for use in a microscope, theapparatus comprising: a transmission-illumination optical system havinga light source, two condenser lenses for a high magnification and a lowmagnification, for collecting light emitted from the light source andilluminating a sample and a shield; and an observation optical systemincluding different objective lenses for observing the sample, having ahigh magnification of a range of ×10 to ×100 and a low magnification ofa range of ×1.25 to ×4, the shield being disposed at or near a pupilposition of the condenser lens in a low-magnification mode thereof,which is conjugate with a pupil position of the objective lens, andcontrolling a shape of art aperture formed within a pupil of theobjective lens; and the condenser lenses being switchable with eachother in accordance with a low magnification and a high magnification ofthe objective lens, wherein the condenser lenses are switched to satisfya condition, NA2/NA1<0.6, where NA1 is a maximum illuminable numericalaperture of the condenser lens and NA2 is a numerical aperture of theobjective lens at which a maximum illumination range of the condenserlens is observed.
 13. A transmission-illumination apparatus for use in amicroscope, the apparatus comprising: a transmission-illuminationoptical system having a light source, two condenser lenses for a highmagnification and a low magnification, for collecting light emitted fromthe light source and illuminating a sample and a shield; and anobservation optical system including different objective lenses forobserving the sample, having a high magnification of a range of ×10 to×100 and a low magnification of a range of ×1.25 to ×4, the shield beingdisposed at or near a pupil position of the condenser lens in alow-magnification mode thereof, which is conjugate with a pupil positionof the objective lens, and controlling a shape of an aperture formedwithin a pupil of the objective lens; and the condenser lenses beingswitchable with each other in accordance with a low magnification and ahigh magnification of the objective lens, wherein the shield isshiftable in accordance with a low magnification and a highmagnification of the objective lens, to satisfy a condition, D2/D1≦0.5,where D1 is an area of the pupil of the objective lens and D2 is an areaof an aperture portion created in the pupil, of the objective lens.